The present invention relates plasma processing apparatuses and methods, and power systems for plasma processing apparatuses. Plasma processing apparatuses are used to etch and deposit different materials in industries such as the semiconductor industry. For example, in a typical dry etching process, a semiconductor wafer including a layer to be etched is disposed within an etch chamber. A gaseous plasma is created in the chamber using radio frequency (RF) power at a single fixed frequency. The created plasma passes through an etch mask on the layer to etch it and form a desired pattern.
It is desirable to increase the number of controllable processing parameters in a plasma processing apparatus. More controllable process parameters provide process engineers with more ways to control the operation of the apparatus to optimize a given process. One way to increase the number of parameters is to use two separate, fixed frequency power sources instead of one fixed frequency power source in the plasma processing apparatus. Power signals at two different fixed frequencies can be applied to a reactant gas in the processing chamber to form a plasma instead of power at one frequency. To further increase the number of process parameters, the duty cycles of the power signals can be modulated. Duty cycle modulation is described in detail below.
FIG. 1 shows a conventional plasma reactor apparatus with two, high power, fixed frequency power sources 21, 24 coupled to a chamber 10. An upper electrode 12 forms part of the chamber 10 and is grounded. One source 21 generates a high power, high frequency RF signal at, e.g., 13.56 MHz. The other source 24 generates a high power, low frequency RF signal at, e.g., 100 KHz. The high and low frequency signals pass from the power sources 21, 24 through respective impedance matching networks 23, 26, and to central and lower electrodes 13, 15. The lower electrode 15 forms part of a wafer chuck which supports a wafer to be processed.
In the apparatus, a control means 27 controls two modulators 22, 25. The two modulators 22, 25 modulate the RF signals coming from the high and low frequency power sources 21, 24 before being sent to the electrodes 13, 15. RF signals from the respective power sources 21, 24 are modulated to provide a duty cycle of 100% or less. xe2x80x9cDuty cyclexe2x80x9d refers to the fraction, expressed as a percent, per unit time that a signal is xe2x80x9conxe2x80x9d or applied to a load. For example, a signal that is applied to a load one-half the time has a fifty percent duty cycle.
Duty cycle modulation is a type of xe2x80x9cpulse modulationxe2x80x9d and can be described in greater detail with reference to FIG. 2. FIG. 2 illustrates a pair of waveforms representing the control signals from the control means 27 to the modulators 22, 25. Waveforms 31 and 32 represent the signals to modulators 22 and 25, respectively. As illustrated in FIG. 2, a process is initiated at a time A and the 13.56 MHz signal from source 21 is continuously applied to electrode 13 until the process is discontinued at time B. Meanwhile, the control signal 32 is supplied to the modulator 25 to turn the 100 KHz signal from the source 24 on and off at some suitable rate. In this example, the duty cycle of signal 31 is 100% and the duty cycle of signal 32 is approximately 50%.
The matching networks 23, 26 in the apparatus reduce the amount of power reflected back to the RF power sources 21, 24. During processing, the plasma within the chamber forms an impedance (capacitive reactance) which changes. An RF mismatch arises between the power sources 21, 24 and the plasma so that some of the RF power is reflected back to the power sources 21, 24. The matching networks 23, 26 compensate for the changing plasma impedance. Each matching network 23, 26 is adapted to minimize power reflections at the frequencies of the power sources 21, 24. For example, a power source providing power at 100 MHz uses a matching network designed to reduce reflections of power at about 100 MHz. If too much power is reflected back to the power sources 21, 24, the power sources 21, 24 may be detrimentally driven into foldback.
While the described apparatus is suitable for some applications, many improvements could be made. For example, it would be desirable if the apparatus was less complex, and consequently less costly to manufacture. The apparatus has two different matching networks 23, 26 for two different high power, fixed frequency power sources 21, 24. Each matching network 23, 26 includes a number of components such as variable capacitors, electric motor servos, and an RF detector circuit. In addition to being complex, many of the components of the matching networks 23, 26 are bulky and can fail. Also, an apparatus using two high power sources 21, 24 is more complex and consumes more energy than an apparatus using one high power, fixed frequency power source. Increased energy consumption leads to increased operating costs.
Also, when using two high power fixed frequency power sources in the apparatus, the power sources and matching networks can interfere with each other. For example, in some conventional apparatuses, two separate different fixed frequency power sources deliver different frequencies of power to the same electrode (e.g., the wafer chuck). In this situation, the two fixed power source/matching network combinations are electrically coupled to each other. When fixed frequency power is applied to the chamber by one power source, the other power source and its associated matching network can act as a load to the other power source. Consequently, some power can be inadvertently diverted from one power source/matching network combination to another combination instead of to the reactant gas in the chamber. The interference between different power sources and matching networks can be reduced by decoupling electronics. However, using decoupling electronics in the apparatus further increases the cost and the complexity of the apparatus.
Also, when two high power, fixed frequency power signals are delivered to a chamber, they can interfere with each other and cause arcing. Arcing can cause instabilities in the plasma and can cause undesired process variations. Arcing can be avoided by limiting the gas pressure range in the chamber, reducing the total RF power, or reducing the low frequency power applied to the chamber. Although the likelihood of arcing can be reduced by limiting the range of process parameters such as these, it is sometimes desirable to enable broader ranges of such process parameters to be used without inducing arcing.
In addition, while the apparatus can deliver one or two fixed frequency, duty cycle modulated power signals to the chamber, the apparatus has a relatively small number of adjustable process parameters. For example, the apparatus only provides for a) one or two fixed frequencies of power to a chamber, and b) the ability to turn the power on and off at some rate. While the ability to modulate the duty cycle of a power signal provides some ability to control the plasma process, the power provided by a duty cycle modulated power signal is still at a single frequency and amplitude. Consequently, the controllable variability of the plasma process is limited.
Embodiments of the invention address these and other problems.
One embodiment of the invention is directed to a plasma processing apparatus. The apparatus comprises: a single carrier source adapted to generate a first RF signal at a carrier frequency; a modulation source adapted to generate a second RF signal at a modulation frequency; a modulator adapted to modulate the first RF signal with the second RF signal to form an amplitude modulated signal, wherein the amplitude modulated signal contains peaks with amplitudes greater than or less than amplitudes of the peaks of the first RF signal; and a plasma processing chamber coupled to the modulator.
Another embodiment of the invention is directed a plasma processing apparatus. The apparatus comprises: a carrier source adapted to generate a first RF signal at a carrier frequency; a modulation source adapted to generate a second RF signal at a modulation frequency; a modulator adapted to modulate the first RF signal with the second RF signal to form a frequency modulated signal; and a plasma processing chamber coupled to the modulator.
Another embodiment of the invention is directed to a method of delivering a power signal to a plasma processing chamber. The method comprises: generating a first RF signal at a carrier frequency; generating a second RF signal at a modulating frequency; forming an amplitude modulated (AM) signal by modulating the first RF signal with the second RF signal, wherein the amplitude modulated signal contains peaks with amplitudes greater than or less than amplitudes of peaks of the first RF signal; and delivering only the amplitude modulated signal to a reactant gas within the plasma processing chamber to form a plasma.
Another embodiment of the invention is directed to a method of delivering a power signal to a plasma processing chamber. The method comprises: generating a first RF signal at a carrier frequency; generating a second RF signal at a modulation frequency; forming a frequency modulated (FM) signal by modulating the first RF signal with the second RF signal; and generating a plasma within the plasma processing chamber using the frequency modulated signal.
Yet other embodiments of the invention are directed to power systems for plasma processing apparatuses. The power system embodiments may include a carrier source adapted to generate a first RF signal at a carrier frequency; a modulation source adapted to generate a second RF signal at a modulation frequency; and a modulator adapted to modulate the first RF signal with the second RF signal to form a frequency and/or an amplitude modulated signal
These and other embodiments of the invention are discussed in further detail below with reference to the Figures and the Detailed Description.